Paul that that the good he does is "Christ in him"Early in Paul's Christian life he was confused, he said, Rom. 7:21, that when he wanted to good, he found evil was present with him, he is referring to the sin nature. In 19--20 he is fighting a losing battle with this sin nature.
In vs 24 Paul asks the question, who shall deliver me from this body of death? He's referring to the sin nature he's fighting a losing battle with ruling over the good he wants to do. He asks the question, who shall deliver me from this body of death? The sin nature is pulling him away from God.
Then in vs. 25, Paul presents the answer to his own question, "I thank God through Jesus Christ our Lord." The answer is found in Christ.
Continuing in vs 25 he identifies the sin nature as the flesh serving the Law of Sin, but with the mind he serves the Law of God.
Here is the struggle we all face in this Christian life with the sin nature. We are presented with the choice of following Christ and therefore receiving the help of the Holy Spirit to overcome the sin nature, or not having the relationship with Christ and serving the Law of Sin that leads to spiritual death and separation from God.
There are two options (biblically) - flesh and spirit. Adam was created flesh, and we were born flesh. Flesh cannot please God.
The Word was God and was with God. God is Spirit. But the Word became flesh. Flesh-Spirit....100%man - 100% God.
We do struggle between the flesh and spirit where those not born of the Spirit don't. Jesus was tempted by the flesh but submitted all things to the will of the Father.
No 3rd type of "nature" needed for us to sin. The desires of the flesh absent the will of the Spirit equates to sin every time.